Deep-submicron Complementary Metal-Oxide-Semiconductor (CMOS) is the primary technology for ultra-large scale integrated (ULSI) circuits. Scaling of CMOS has been the principal focus of the microelectronics industry over the last two decades. As device sizes are scaled down, the source and drain junctions have to scale down accordingly to suppress the so-called short channel effects (SCE) which degrade the performance of scaled MOS devices. A major problem related to source and drain junctions is that the source and drain series resistance increases as the devices are scaled down, which degrade the device performance.
Advanced MOS transistors today have shallow source and drain junctions, which consist of shallow source and drain extension junctions to suppress SCE, and relatively deep source and drain junctions to improve series resistance. The resistance from shallow source and drain extension junctions is a large portion of the total series resistance. As extension junctions become shallower and shallower, the series resistance tends to become greater and greater. In order to make the source and drain series resistance low, high doping density is needed in the shallow extension junctions.
Generally, the shallow source and drain extension junctions and the relatively deep source and drain junctions are implemented by ion implantation of dopant through a screen oxide (Si0.sub.2) layer. Implantation through an oxide layer results in the deposition of recoiled oxygen into the silicon. This oxygen has a tendency to reduce the interstitial silicon which is present in the implant-damaged substrate. This, in turn, suppresses the diffusion tail associated with the implanted dopant, which moves by an interstitialcy mechanism. This increases the series resistance.
For N-channel Metal-Oxide-Semiconductor (NMOS) transistors, Group V elements are used as dopants.
For P-channel Metal-Oxide-Semiconductor (PMOS) transistors, Group III elements are used, with boron (B) being the most commonly used dopant for both the shallow extension junctions and the relatively deep source and drain junctions. Boron is incorporated into the silicon through ion implantation of either boron atoms or boron difluoride (BF.sub.2).
After ion implantation, the dopants need to be activated by a thermal treatment-usually a rapid thermal annealing (RTA). This causes a significant problem in that boron atoms tend to segregate from the silicon substrate into the screen oxide layer during the RTA of dopant activation. Boron segregation results in a decrease of boron concentration in the silicon substrate near its surface and therefore an increase of source and drain series resistance. This effect becomes more and more of a problem as device sizes are continuously scaled down and the extension and junction depths becomes shallower and shallower.